As the dimensions of semiconductor devices have become smaller, the wavelength of the radiation that is used to expose the photoresist has also become smaller. The short-wavelength radiation is required for the resolution that is necessary to define the extremely minute features of these devices. "I-line" radiation with a wavelength (.lambda.) of 365 nm and deep ultraviolet (DUV) radiation with a wavelength of 248 nm are now in use and the introduction of radiation having a wavelength of 193 nm is foreseeable.
The use of short-wavelength radiation has the effect of increasing the reflectivity of the radiation at the interface between the photoresist layer and the underlying material, and this has led to thin film interference (TFI) effects, such as standing waves which produce variations in the dimensions of the features of the device and the exposure of normally unexposed areas from non-normal reflections (sometimes called "reflective notching").
To overcome these problems, semiconductor device manufacturers have turned to the formation of antireflective layers (ARLs) underneath the photoresist. By means of negative interference and absorption, ARLs substantially reduce the amount of radiation that is reflected back into the photoresist layer where it can create the problems referred to above. See generally, De Jule, "Resist Enhancement With Antireflective Coating", Semiconductor International, July 1996, p. 169 et seq.
A layer of silicon oxynitride (SiON) having a thickness in the range of 50 .ANG. to 1 micron has been widely used as an ARL. While SiON has good functional qualities, a problem with SiON is that it can be etched only at a relatively slow rate. In many situations the ARL must be removed after the underlying layer has been patterned. Typically, the process sequence would be as follows. First, after the ARL and photoresist layer have been deposited, the photoresist layer is patterned. Second, the ARL exposed by the patterning of the photoresist is etched. Third, the underlying layer is patterned, using the photoresist as a mask. Fourth, the photoresist layer is removed. Fifth, the ARL is etched.
Silicon nitride (SiN.sub.x) has also been suggested for use as an ARL, in T. P. Ong et al., "CVD SiN.sub.x Anti-reflective Coating for Sub-0.5 .mu.m Lithography", 1996 Symposium on VLSI Technology Digest of Technical Papers, p 73 et seq.
In etching the ARL it is important to minimize the damage to the underlying, patterned layer. The slow etch rate of SiON creates problems in this regard. For example, FIG. 1 shows a layer 10 of borophosphosilicate glass (BPSG) which has been patterned to form an aperture 14 for a contact. The difficulty of etching the ARL 12 has created an overhang of the BPSG layer 10 on either side of the aperture. Certain applications in logic, memory and flash technologies also require that the ARL be removed without doing damage to the underlying structure. FIG. 2 shows a typical floating gate memory transistor 20, including a gate oxide layer 21, a floating polysilicon gate 22, an oxide-nitride-oxide (ONO) layer 23, a control gate 24, an overlying oxide layer 25, and an ARL 26. In such devices, it is important that the gate oxide layer 21, the ONO layer 23 and the oxide layer 25 remain unperturbed after the ARL 26 has been removed. Furthermore, in the fin-type capacitors used in certain DRAM designs, it is desirable to leave the oxide or ONO layer undamaged after the ARL has been etched.
Each device can be considered as having an "etch budget", which is the amount of time during which the exposed structures can be subjected to etchant without undue damage. In many cases the removal of the silicon oxynitride and silicon nitride ARLs that have been developed until now exceeds this etch budget. Thus, a need exists for an ARL which can be removed relatively fast and without exceeding the etch budget.